Aircraft operating conditions provide unique challenges for the design of aircraft fire suppression systems. For example, aircraft fire suppression systems must work at a wide range of temperatures. These temperature may range from +105° C. when the aircraft is on the tarmac on a hot day, to as low as −55° C. when the aircraft is at high altitudes.
For more than 50 years Halon 1301 has been the agent of choice for aircraft engine, auxiliary power unit (APU), and cargo fire suppression applications. Halon 1301 has a number of specific desirable properties that make it a popular choice for aircraft fire suppression systems. For example, Halon 1301 has a low boiling point and a high vapor pressure, which facilitates agent-air mixing and distribution throughout the fire zone. In addition, the −58° C. boiling point of Halon 1301 and its ability to freely vaporize at each point of discharge are desirable physical properties. However, due to the ozone depleting potential of Halon 1301 (Bromotrifluoromethane), manufacturing of the material ceased in most countries in 1995.
In many current systems, Halon 1301 is stored in a pressurized bottle, which uses nitrogen as a pressurizing gas. Nitrogen pressure beyond the natural vapor pressure of Halon 1301 is needed to provide system discharge energy at low temperatures. Nitrogen dissolved in the Halon solution also improves vaporization and breakup of liquid drops of Halon 1301 at low temperature similar to a “popcorn” effect.
Aircraft fire suppression systems are usually designed based on the weight of the agent required to achieve a specific minimum agent concentration in the fire zone immediately after the bottle discharges. The fire suppression system should be designed to function properly at the minimum operating temperature for the application. The minimum operating temperature is often the worst case scenario for the fire suppression system because agent vapor volume and vapor pressure decrease with decreasing temperature.
Another important consideration in the design of the fire suppression system is agent distribution. Agent distribution throughout the fire zone depends on the agent's ability to mix with air entering the fire zone at each discharge location. Presence of clutter in the fire zone may present challenges to the line-of-sight transport between the discharge location and the fire threat.
Currently, there are no known fire suppression and extinguishing compounds that have the characteristics and capabilities of Halon 1301 but are also environmentally friendly.